Today's vote in the House of Commons is a vital opportunity for MPs to grasp the mettle and ensure that children do not suffer because of outdated adoption legislation.

This vote is not about marriage, nor is it a gay rights issue. It is about giving the 5,000 children who are waiting for families the opportunity of a permanent and loving family.

We desperately need to widen the pool of potential adopters so we can find suitable parents for children who are waiting. We cannot emphasise how important this is at a time when, whether we like it or not, the structure of our society is radically changing. One in five households are now headed by an unmarried couple - in coming years, this figure is expected to reach one in three. In this context it is vital that we do not needlessly restrict the categories of people who can adopt.

Of course, we want to make sure unmarried couples who care for a child have a stable and permanent relationship, but we know couples who choose not to marry can still demonstrate this commitment.

Some MPs point to the high rate of breakdown in cohabiting relationships, but these are not the people who will get through the first steps of the adoption process. Unmarried couples who are approved as adopters will have gone through rigorous checks. As with married couples social workers will have to consider the length of time a couple have lived together, how well they know each other and whether they intend to make a lifelong commitment.

Undoubtedly we will sometimes get it wrong and couples will break up - at a time when one in three marriages founder it will be surprising if some cohabiting relationships do not - but the tests for adopters are rightly rigorous. All we are asking for is that unmarried couples be given the right to apply to adopt jointly, so they can go through the assessment procedure. Ultimately, it will be up to the courts to decide whether a placement is in the best interests of the child.

In reality some single people who are living with a permanent partner do already adopt but in theses cases the child only has a legal relationship with one of the couple.

Children have a right to have the same legal relationship with both the adults caring for them on a day to day basis. What if the parent dies? What if an important consent form needs urgent signing? Do we really want to leave children vulnerable to an uncertain outcome?

We know some unmarried people do not come forward as adopters because they know they cannot make the same equal commitment to the child.

A lot has been said about the many married couples who have been approved to adopt and are still waiting for a child to be placed with them. The reality is that many of these couples understandably want to adopt healthy babies to replace the children that sadly, through infertility, they have been unable to have themselves.

But it is a sad reality that half of boys over five years old for whom social workers are seeking families may never find one. What do you say to a child for whom nobody comes forward?

This review of adoption is not a gay rights issue - gay and lesbian people can and do already adopt as single people. If two people living together are jointly parenting a child please let us give the child the benefits of having an equal legal relationship with both carers.

We urge MP's to support today's amendment. Society's most vulnerable children deserve no less.

· Felicity Collier is chief executive of the British Agencies for Adoption and Fostering